Method and device for continuous casting of metals

ABSTRACT

A method and a device for continuous casting of metals, in which the device comprises a mould ( 1 ), through which a liquid metal passes, and a member ( 4 ), through which a liquid metal is supplied as a jet to the mould ( 1 ). The device comprises magnet members ( 6, 7 ) for applying a magnetic field varying with time and being substantially fixed in the room to the liquid metal in the mould ( 1 ).

BACKGROUND OF THE INVENTION AND PRIOR ART

The present invention relates to a method for continuous casting ofmetals, in which a liquid metal is supplied as a jet to a mould, whichalready contains further liquid metal. The invention also relates to adevice for continuous casting of metals, comprising a mould, throughwhich a liquid metal passes, and a member through which a liquid metalis supplied as a jet to the mould, in which there is already liquidmetal.

In continuous casting a liquid metal is supplied to a mould, in which itis cooled and shaped to an elongated string. Depending upon the crosssection dimensions the string is called “billet”, “bloom” or “slab”. Aprimary flow of hot, liquid metal is during the casting delivered to acooled mould, in which a metal is cooled and at least partiallysolidifies into an elongated string. The cooled and partially solidifiedstring leaves the mould continuously. At the point where the stringleaves the mould it has at least a mechanically self-supportingsolidified skin surrounding a centre part not solidified. The cooledmould is open at two opposite ends as seen in the casting direction andpreferably connected to means for supporting the mould and means forsupplying cooling means to the mould and the support means. The mould ispreferably made of an alloy with a copper base and a high thermalconductivity.

The liquid metal is supplied to the mould from a casting box through atube extending down into the mould. The tube extends preferably that farinto the mould that it projects into the liquid metal preferably presentthere. When the liquid metal from the tube flows into the liquid metalalready present in the mould, it generates a so-called primary flow anda so-called secondary flow. The primary flow goes downwardly in thecasting direction, while the secondary flow goes from the region of thewalls of the mould upwardly towards the surface of the metal bathlocated therein and downwardly. In different parts of the metal bathpresent in the mould periodic velocity oscillations are created duringthe casting sequence. Thus, upper and lower loops in which the liquidmetal flows around are formed in a way known per se. As a consequence ofresonance phenomena, which are associated with periodical oscillationsof such loops, large bubbles, for instance argon bubbles, oxideenclosures from the casting tube and slag from the meniscus will betransported far downwardly in the casting direction, i.e. far downwardlyin the casting string initially formed in the mould. This results inenclosures and irregularities in the final, solidified casting string.

Velocity variations caused by oscillating flow in the mould gives riseto pressure variations at the meniscus, and meniscus height variations.At high meniscus velocities, this results in

(a) draw-down of slag,

(b) uneven slag thickness,

(c) uneven skin thickness, and

(d) a risk of crack formation.

Furthermore, the oscillating flow results in an asymmetric velocitydownwardly in the mould. The velocity may in some positions at onenarrow side get substantially higher than at the other. This gives riseto a strong transport downwardly of enclosures and gas bubblesaccompanied by a decreased slag quality.

The prior art relates to different devices and methods for influencingthe primary and the secondary flows, respectively, in the liquid metalin the mould. The prior art utilizes for this devices for applyingsubstantially static magnetic fields during the casting sequence over atleast a part of the liquid metal contained in the mould. It is forinstance known through the Swedish patent publication SE 436 251 toarrange a static direct current magnetic field or permanent magneticfield at the mould. It may as an a alternative be formed by a lowfrequency alternating current field having a frequency below 1 Hz. Whenthe metal flowing in passes this field the movement of the tap jet intothe rest of the liquid metal is retarded, wherethrough the flow pictureis influenced favourably for the casting sequence. This technique hasthen been further developed. The magnets used for the generation of themagnetic field have for example been placed so that a magnetic field atdifferent levels of the mould in the casting direction has beenobtained, whereby specific local movements in the liquid metal could beinfluenced separately through the respective magnetic field. It has alsobeen proposed to arrange the magnets and the yokes connecting them insuch a way that the magnetic fields extend in the casting direction instead of transversely thereto.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method substantiallydisturbing and thereby reducing the generation of periodicaloscillations and resonance phenomena associated therewith, which arecommonly existing in a liquid metal in a mould in connection withcontinuous casting of metals.

This object is obtained by means of a method of the type defined in theintroduction, which is characterized in that a magnetic field varyingwith time and being substantially fixed in the room is applied by meansof magnet members to the liquid metal in the mould for preventingnatural oscillations of the liquid metal from being generated. The fieldwill function as a damper in the liquid metal.

According to a preferred embodiment of the method the application of themagnetic field is carried out periodically. The periodicity may beadapted to the periodicity of the oscillations, which normally forcasting steel slabs is in the order of 1-30 seconds. The field isapplied periodically for disturbing and eliminating oscillations in theliquid metal without changing the main flow topology therefor. The flowtopology may be changed, for example by means of strong, static magneticfield or a moving field.

According to a further preferred embodiment of the method the varyingmagnetic field is applied at irregular intervals during the castingsequence. Thanks to the irregularity of the application it is avoided toregularly amplify certain regular, periodical oscillations in the liquidmetal. The varying magnetic field causes instead thanks to theregularity thereof a disturbance of such regular natural oscillations inthe liquid metal.

According to a further preferred embodiment the irregular application ofthe magnetic field is carried out at random times. The magnetic fieldapplied randomly counteracts and disturbs efficiently the generation ofperiodical oscillations. The random application of the magnetic fieldresults in a minimum risk of possibly amplifying any natural oscillationpresent in the liquid metal during any extended time.

According to an alternative embodiment of the method the periodicalapplication of the magnetic field is carried out at predetermined times.These times are preferably times known in advance, at which periodicaloscillations in the liquid metal are in a certain critical stage, forinstance when resonance phenomena caused by said natural oscillationsstart or may start to occur. The predetermined times are then based onpractical observations or calculations of the time for the occurrence ofsuch critical stages in the liquid metal under given casting conditionsor measurements of meniscus deformation.

According to a further preferred embodiment of the method theapplication is carried out upon detection of a certain state in theliquid metal. Said state is preferably a predetermined detectablemovement in the liquid metal or as an alternative of the meniscus.

According to a further preferred embodiment the varying magnetic fieldis provided with a stochastically varying amplitude. The probability fordisturbing and not amplifying the natural oscillations generated in theliquid metal during the casting sequence is thereby increased.

According to a further preferred embodiment the varying magnetic fieldis given a frequency being in the order of 10-10³ times higher than thefrequency of the oscillation or oscillations in the melt intended to bedisturbed thereby. These are normally of the type wide-band spectrum. Avery reliable disturbance of said oscillations of the liquid metal isthereby obtained. The magnetic field may then advantageously be appliedonly during a restricted part of the period of the oscillation oroscillations and nevertheless give a satisfying and reliable disturbanceinfluence.

A further object of the invention is to provide a device, by means ofwhich natural oscillations and resonance phenomena associated therewithof a liquid metal in the mould during continuous casting of metals maybe disturbed and prevented from being generated.

This object is obtained by means of a device of the type defined in theintroduction, which is characterized in that it comprises magnet membersfor applying a magnetic field varying with time and being substantiallyfixed in the room to the liquid metal in the mould. Thanks to the factthat the magnetic field varies it may easily be controlled, i.e. begiven such an amplitude and frequency, that the periodical oscillationsexisting or generated in the liquid metal are efficiently disturbed.

According to a preferred embodiment the magnet members are arranged insuch a way that they generate the varying magnetic field periodicallyduring the casting sequence. The periodicity is advantageously adaptedto the periodicity of the oscillations, which is in the order of 1-30seconds when casting steel slabs.

According to a further preferred embodiment the magnetic members areadapted to generate the varying magnetic field at irregular intervals.This disturbs efficiently a generation of each natural oscillation,since they will with a high probability be in off-phase with theperiodical oscillations existing in the liquid metal.

According to a further preferred embodiment the magnet members areadapted to generate a magnetic field having a stochastically varyingamplitude. They are preferably also adapted to generate a magnetic fieldhaving a varying frequency, for example stochastically varyingfrequency, within a given frequency interval. Thanks to the variation ofthe amplitude and/or frequency of the magnetic field, or the currentused to generate the magnetic field, a very reliable disturbance of thenatural oscillations in the liquid metal is obtained, at the same timeas an amplification of possibly natural oscillations in the liquid metalare efficiently avoided.

According to a further preferred embodiment the magnet members areadapted to generate a substantially static magnetic field, onto whichthe varying magnetic field is superposed. The static magnetic field ispreferably used for influencing the so-called primary flows andsecondary flows in the liquid metal in the mould, which the liquid metalsupplied thereto gives rise to. The same type of basic equipment, whichhas according to the prior art been used for achieving such aninfluence, may accordingly advantageously be used for the combinedfunction aimed at, since both a static magnetic field and varyingmagnetic field superposed thereon are applied to the liquid metal in themould.

According to a further preferred embodiment of the device the varyingmagnetic field has a frequency in the order of 10-10³ times higher thanthe frequency of the oscillation or the oscillations of the liquid metalto be disturbed thereby. A very reliable disturbance of the naturaloscillations in the liquid metal is thereby obtained.

Further characteristics and advantages of the invention appear from theother dependent claims and the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described as an example withreference to the appended drawings.

In the drawings:

FIG. 1 is a schematic cross section view from one side of a device forcontinuous casting of metals,

FIG. 2 is a cross section view from the side of an upper part of thedevice according to FIG. 1,

FIG. 3 is a view from above of the device according to FIGS. 1 and 2,

FIG. 4 is a diagram showing a static magnetic field, or the directcurrent used for generating this field, on which a varying magneticfield is superposed,

FIGS. 5-7 are diagrams showing how a varying magnetic field, or thecurrent varying said magnetic field, may be varied in the time accordingto different embodiments of the invention and

FIG. 8 is a schematic lateral cross section view showing a device havingmagnet members at only one level.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1-3 show a device for continuous casting of metals, such as forexample steel. The device comprises a mould, which defines a cylindercomposed of four opposite walls. The mould is preferably made of acopper alloy or any other alloy having a suitable thermal conductivityand thermal resistance. Outside at least two opposite walls of the mould1 members 2, 3 for cooling said walls are arranged. The cooling members2, 3 may comprise any type of supporting frame, through which coolingchannels for transport of the cooling medium, such as water, may bearranged. Such channels may possibly be arranged so that they allow thecooling medium to flow directly towards the external surfaces of saidwalls for cooling thereof.

The device also comprises a member 4, here a so-called “submerged entrynozzle, SEN”, through which a liquid metal is supplied from a containernot shown to the space defined by the mould. During the casting sequencethe mould 1 is filled with metal. This metal is present mainly as aliquid metal, but along the interface to the mould 1 it defines asolidified or partially solidified outer layer, a so called skin, whichgrows gradually in thickness in the casting direction. The mould 1 isthen adapted to carry out a substantially vertical oscillating movementwhile a casting string 5 consisting of liquid metal and the solidifiedouter layer surrounding this successively is fed out from one end of themould 1.

The member 4 extends from above into the liquid metal present in themould 1. Further liquid metal is supplied to the mould 1 through themember 4. The liquid metal flowing into the liquid metal in the mould 1from the member 4 generates a primary flow, which is illustrated throughthe arrows 18, and a secondary flow, illustrated through the arrows 19,in the liquid metal already present in the casting mould 1. Thesenatural oscillations are of the type wide-band spectrum. Since resonancephenomena occur as a consequence of these oscillations in the liquidmetal, it may result in the transport of large bubbles, which containgases or slag, downwardly into the casting string 5. As a result thereofenclosures, bubbles or argon containing enclosures and slag from themeniscus are present in the finally entirely solidified casting string5. Velocity variations caused by the oscillating flow in the mould maygive rise to pressure variations at the meniscus and height variationsof the meniscus. This leads at high meniscus velocities to slagdrawdown, an uneven slag thickness, an uneven skin thickness and therisk of crack formation. Furthermore, the oscillating flow results in anasymmetric velocity in the mould. The velocity at one narrow side may incertain positions be substantially higher than at the other narrow side.This results in a powerful transport downwardly of the enclosures andgas bubbles accompanied by a decreased slag quality.

For avoiding the problem mentioned above the device comprises a firstset of magnet members 6, which are adapted to apply or generate avarying magnetic field across the liquid metal in the mould 1, i.e.transversal to the casting direction. The magnet members are accordingto a preferred embodiment adapted to irregularly, e.g. at random timesbut within given limits in the time, generate a varying magnetic fieldwith a random amplitude within at least a predetermined amplitude range.

The magnet members may also advantageously be designed to generate themagnetic field in such a way that it has a frequency varying randomly,in which also the frequency is within at least a predetermined range.The lowest frequency, or at least the average frequency, should however,exceed 1 Hz, and preferably exceed 10 Hz. As an alternative, the magnetmembers may be arranged in such a way that they generate a magneticfield with a determined amplitude and frequency, for example a magneticfield which may be described by a square wave or a sinus-shaped wave,such as in FIGS. 6 and 7. The device may also comprise means (not shown)for activating the magnet members to generate said magnetic field atpredetermined periods. Said activating means may then comprise means 20for detecting a certain condition or for predicting the appearance of acertain condition by modelling/calculation, for example a certainmovement in the liquid metal or of a member 4, in which activation ofthe magnet members is to be carried out and means 21 for controllingsaid activation. A device adapting the disturbance by naturaloscillations existing thereof to the conditions prevailing for themoment in the liquid metal is thereby obtained.

The device also comprises a second set 7 of magnet members. This secondset 7 is arranged closer to the upper surface of the liquid metal in themould 1 than the first set 6 and is arranged substantially on the samelevel as that part of the member 4 that projects down into the liquidmetal. The first set 6 of magnet members is arranged immediately afterthe member 4 as seen in the casting direction, in this case below theopening or openings of the member 4 for letting the liquid metal out.Also the second set 7 of magnet members is advantageously arranged togenerate a magnetic field similar to any of the magnetic fieldsdescribed above and generated by the first set 6 of the magnet members.

FIG. 3 shows a set of magnet members 6 or 7, seen from above. As appearsfrom FIG. 3, the set 6, 7 comprises two couples of magnet cores,preferably iron cores 8, 9 and 10, 11. On each of the said cores 8-11 awinding 12-15 of an electric conductor is arranged. Said windings 12-15are fed from one or several (not shown) current sources with current,preferably direct current varying in time, for generating a magneticfield extending across, i.e. through, the liquid metal in the mould 1.The cores 8, 9 and 10, 11 of the respective couple are connected to eachother through yokes or leg members 16, 17. The magnet cores and thewindings are arranged in a way known per se, as appears from FIG. 3, butthey could of course be arranged in another way and possibly comprisemore or fewer individual magnet cores and windings arranged thereon.Thus, second arrangement of yoke members 16, 17 and magnet cores 8-11with respect to each other is possible, for example for generating themagnetic field on several levels in the casting direction, or forgenerating one or several magnetic fields extending in the castingdirection.

According to prior art the magnet members have, arranged in the wayappearing from FIGS. 1-3, been adapted to generate a generally static orperiodic low-frequency (f<1 Hz) magnetic field across the liquid metalin the mould 1 in order to influence the primary and secondary flowsdescribed earlier in the liquid metal. Problems associated with saidflows and described within the prior art are thereby solved. The sets 6,7 of magnet members in the device according to the present invention arepreferably also arranged to generate a substantially static magneticfield across the liquid metal in the mould 1 to influence the primaryand secondary flows in a way favourable for the casting sequence. Thedifferent types of varying magnetic fields generated by the sets ofmagnet members 6, 7 are superposed onto said static magnetic field. Thestatic magnetic field is shown in FIG. 4, while different types ofvarying magnetic fields, which may be superposed onto the staticmagnetic field, are shown in FIG. 5-7. The varying magnetic fields shownin FIGS. 5-7 are examples of how such magnetic fields may be generatedaccording to the invention. The magnetic field according to FIG. 5 isgenerated at random times, has a stochastic amplitude within a givenrange, as well as a stochastic frequency within a given range. Themagnetic fields according to FIG. 6 have, however, a given constantamplitude and frequency, which is also the case for the magnetic fielddescribed through FIG. 7. The diagrams shown in FIGS. 4-7 may also bedescribed to show the current applied to the conductor windings 12-15for generating said magnetic fields.

The frequency of the varying magnetic field is preferably higher thanthe frequency of the oscillation or the oscillations of the liquid metalto be disturbed by said magnetic field. The frequency of the magneticfield is preferably in the order of 10-10³ times higher than thefrequency of said oscillations. The oscillations have a frequency in theorder of 0.01-10 Hz. Since the frequency, or the average frequency, ofthe magnetic field is lower or of substantially the same size as saidfrequencies, it should be generated so that it is in opposite phase, orat least not entirely in phase with said oscillations.

In some cases, for example when natural oscillations having differentamplitudes and frequencies are present in different parts of the liquidmetal, the magnet members are advantageously adapted to generate one orseveral magnetic fields adapted to the specific oscillations conditionsprevailing in the different parts of the liquid metal where exactlythese magnetic fields propagate.

FIG. 8 shows an alternative embodiment of the device according to theinvention, where only one set of magnet members 22 is arranged at onlyone level at the mould in the casting direction. The set 22 is arrangedin a region downstream of the opening/openings of the member 4.

A plurality of modifications of the device according to the inventionand the method according to the invention will of course be apparent toa man skilled in the art while these are still within the scope ofprotection of the present invention, such as this is defined in theappended claims.

It is important to note that the magnetic field is fixed or stationary,i.e. it does not move and thereby agitate the liquid metal, which is thecase for conventional agitators.

It is to be understood that the member 4 may be arranged in such a waythat it does not project into the liquid metal in the mould, by which afree tap jet reaches the liquid metal.

It should also be noticed that the natural oscillations, which theinvention aims to damp or disturb, comprise large movements within theliquid metal, where large portions in the liquid metal with certainflowing conditions are displaced mutually more or less periodically,whereby unfavourable casting conditions occur.

The varying magnetic field is preferably superposed onto a substantiallystatic magnetic field normally being constantly applied to the liquidmetal. By applying the varying magnetic field in pulses or periodicallythe strength of the static field may be reduced somewhat at leastperiodically, while establishing desired casting conditions. This may bean advantage with respect to the flow topology as well as the energyconsumption.

What is claimed is:
 1. A method for continuous casting of metals, inwhich liquid metal is supplied as a jet to a mould which alreadycontains a further liquid metal, wherein: a magnetic field varying withtime and being substantially stationary is applied by magnet members tothe liquid metal in the mould for preventing natural oscillations of theliquid metal from being generated, and the varying magnetic field isapplied at irregular intervals during the casting sequence.
 2. A methodaccording to claim 1, wherein a substantially static magnetic field isapplied to the liquid metal, and said varying magnetic field issuperposed onto the substantially static magnetic field.
 3. A methodaccording to claim 1, wherein the application of the varying magneticfield is carried out periodically.
 4. A method according to claim 3,wherein said periodic application is carried out at predetermined times.5. A method according to claim 1, wherein said irregular application iscarried out at random times.
 6. A method according to claim 1, whereinthe application is carried out upon detection of a predeterminedcondition in the liquid metal.
 7. A method according to claim 1, whereinthe varying magnetic filed is provided with stochastically varyingamplitude.
 8. A method according to claim 1, wherein the varyingmagnetic field has a constant peak of height.
 9. A method according toclaim 1, wherein the varying of the magnetic field defines asinus-shaped wave.
 10. A method according to claim 1, wherein thevarying magnetic field is provided with a frequency which is in an orderof 10-10³ times higher than the frequency of the oscillation oroscillations which it is intended to disturb.
 11. A method forcontinuous casting of metals, in which liquid metal is supplied as a jetto a mould which already contains a further liquid metal, wherein: amagnetic field varying with time and being substantially stationary isapplied by magnet members to the liquid metal in the mould forpreventing natural oscillations of the liquid metal from beinggenerated, and the varying magnetic field is provided withstochastically varying amplitude.